Midstory removal reduces effectiveness of oak (Quercus) acorn dispersal by small mammals in the Central Hardwood Forest region

Timber harvests that aim to promote oak (Quercus) regeneration may have indirect impacts on seedling recruitment by altering trophic interactions between oak and animals. For example, changes in habitat structure following harvest may alter the conditionally mutualistic relationship between oak and small mammal granivores like the gray squirrel (Sciurus carolinensis), eastern chipmunk (Tamias striatus) and white-footed mouse (Peromyscus leucopus) that act both as acorn predators and as dispersers. Over 4 years (2010-2013), we tracked the fate of a total of 8000 tagged acorns placed beneath black (Quercus velutina) and white oaks (Quercus alba) at 5 control (unharvested) sites and 5 sites where the midstory was removed as part of a three-phase shelterwood harvest in an oak-dominated forest in southern Indiana, USA. We found that acorns in harvested sites were more likely to be taken by granivores and were less likely to survive the winter, resulting in an overall 67% reduction in seed dispersal effectiveness (SDE). Harvested sites had more vegetative cover and coarse woody debris in the understory, providing increased cover from aerial predators and potentially facilitating greater acorn removal and a higher probability of cache recovery. Reduced seed dispersal effectiveness following midstory removal has potential to slow accumulation of oak regeneration prior to overstory removal and therefore to reduce the effectiveness of shelterwood treatments in promoting future oak dominance in harvested stands. Augmentation of shelterwoods with prescribed fire or other treatments to reduce understory density may counter this effect.